The present invention relates to reflective micro-mirrors and micro-mirror arrays for, e.g., projection-type displays, for steering light beams, maskless lithography, maskless micro-array production, etc. A common feature is a micro-mirror that is movable so as to deflect incident light. In one type of conventional direct view or projection-type display system, an array of reflective micro-mirrors is provided for producing an image. Typically the micro-mirrors have a either a tilt angle for the “ON” state and a non-deflected resting state for the “OFF” state, as shown in FIG. 1, or the same tilt angles for “ON” and “OFF” states but opposite sign (not shown).
Referring to FIG. 1, micro-mirror 110 operates at binary states—“ON” and “OFF” states. The “OFF” state is defined as micro-mirror 110 being placed horizontally. And the “ON” state is defined as micro-mirror 110 being rotated at an angle θ from the “OFF” state. In operation, micro-mirror 110 switches between the “ON” and “OFF” states, thus selectively reflecting light into or away from target 130 (e.g. a screen or a viewer's eyes).
As an exemplary application for displaying an image, micro-mirror 110 is associated with a pixel of an image. At a time when the pixel is “bright”, micro-mirror 110 is set to the “ON” state. Incident light 120a is reflected as light 120b, generating an image profile 140a of micro-mirror 110 on target 130. At another time when the pixel turns to “dark”, micro-mirror 110 is set to the “OFF” state. Incident light 120a is reflected away from target 130, resulting in an image profile 140b in a different location other than the target 130.
When the tilt angle θ is small, the two image profiles 140a and 140b may be overlapped as shown. Target 130 collects reflected light from both “ON” and “OFF” states. When target 130 is a pixel of a displayed image, the contrast ratio of the pixel, thus the quality of the displayed image is reduced. Contrast ratio is the ratio of luminance between the brightest white that can be produced and the darkest black that can be produced. And it is a major determinant of perceived image quality. If a displayed image has high contrast ratio, a viewer will judge it to be sharper than a displayed image with lower contrast ratio, even if the lower contrast image has substantially more measurable resolution. The degradation of the contrast ratio becomes larger as the angular extent of the incident beam grows.
Therefore, a micro-mirror with high contrast ratio is desired.